In general, paraffinic and naphthenic oils and high aromatic oils have so far been used as a softening agent for a rubber composition and as an extender oil for a synthetic rubber, and they have been used properly depending on the uses thereof. In particular, high aromatic oils have preferably been used as a softening agent for a rubber composition and as an extender oil for a synthetic rubber because of the following reasons (1) to (4):
(1) high aromatic oils have a characteristic of giving a high loss performance (high heat generating property) to a rubber composition, PA1 (2) an affinity thereof with a rubber component, which is natural rubber or synthetic rubber, contained in a rubber composition, is the best, PA1 (3) high aromatic oils have a softening characteristic for improving the workability of a rubber composition, and PA1 (4) the profitability is excellent (inexpensive).
It has been advised, however, in PROPOSITION-65 enforcement in 1988 and a series of EU instructions started in 1989 that the high aromatic oils have a possibility to be a carcinogen because of polycyclic aromatic compounds [hereinafter, called a DMSO extract or PCA (polycyclic aromatic compounds) contained as components in the high aromatic oils, and therefore a high aromatic oil having no problem from the viewpoint of safety and hygiene as well as the characteristics (1) to (4) described above has been desired.
Following the series of regulations described above, in particular, the EU instructions, several proposals have so far been made from the viewpoint of a rubber composition. At present, however, any high aromatic oils which satisfy the three required characteristics described above and concurrently are adequate from the safety and hygiene point of view have not yet been proposed, much less rubber compositions or oil extended synthetic rubbers using said high aromatic oil.
A synthetic rubber obtained by using, as an aromatic monomer, a high aromatic lubricant which has a viscosity of 32 to 50 cst and which contains an aromatic component of 30 to less than 55% by weight and a saturated hydrocarbon component of 40 to 65% by weight, both of which are measured by Clay-Gel method (ASTM D 2007), and a polycyclic aromatic compound (PCA) of less than 3% by weight measured according to IP (The Institute of Petroleum) 346 method (PCA standard test) which is a method of extracting oil into dimethylsulfoxide as a means of determining polycyclic aromatic compound content, in particular, a rubber composition obtained by using the above lubricant for SBR in a proportion of 10 to 25% by weight based on the whole weight of the rubber composition has been proposed in, for example, Japanese Patent Application Disclosure No. Hei 6-505524 [British Patent (GB) 2252978, U.S. Pat. No. 5,504,135 and European Patent (EP) 575400].
The viscosity of the high aromatic oil, however, does not have a relation to the characteristic of providing either the high loss performance or the affinity with rubber among the characteristics required to the performances of the high aromatic oils described above and is not univocally related as well to the softening characteristic. In addition, there is the problem that the high aromatic oil can not stand the use as an alternative for conventional high aromatic oils, because the aromatic component falling within the above range, measured by the Clay-Gel method, does not provide the sufficiently high loss performance and the saturated hydrocarbon component falling within the above range does not provide as well the sufficient affinity with rubber.
Further, a method for solving a inferior high loss performance of a high aromatic oil containing less polycyclic aromatic compound (PCA) by making use of an cumaron-indene resin is proposed in West Germany Patent (DE) 3938620 [European Patent (EP) 429761, U.S. Pat. No. 5,091,007 and East Germany Patent (DD) 299193]. This method, however, has problems that the profitability is very bad and that the affinity with rubber is low since the amount of an aromatic component contained in the oil used is as small as 0 to 30% by weight, and therefore the practicability is poor.
Thus, conventional methods are in a state that performances, profitability and safety and hygiene can not stand together, and therefore the development of a novel high aromatic oil satisfying all of performances, profitability and safety and hygiene has strongly been desired.